Abstract

The observed secular period changes of RR Lyrae stars in five globular
clusters have been compared with those predicted by synthetic models of
the horizontal branch (HB). It is shown that most of the observed number
distributions of the period change rate beta could be attributed to
evolutionary effects, if the random observational error is of order + or
- 0.07 days/Myr in beta as suggested by the observers. The model
calculations indicate that the mean rate of period change in RR Lyrae
stars in globular clusters depends sensitively on their HB type, as is
the case for the period shift at a given effective temperature. The
distinct bias toward positive period changes in M15 and Omega Cen is
evidence that most HB stars in the Oosterhoff group II clusters pass
through the instability strip from blue to red toward the end of their
core helium burning phase. If confirmed by future observations, this is
strong support for the Lee et al. (1990) explanation of the Sandage
(1957) period-shift effect.

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